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Journal of Contemporary Brachytherapy (2011/volume 3/number 4)

Address for correspondence: Stefan Strohmaier, MSc, c/o: Dipl. Ing. Peter Tonhuser,

Eckert+Ziegler Bebig GmbH, Berlin, phone: +49 30 941084 297, fax: +49 30 941084 630,) e-mail: [email protected]

Review article

Comparison of 60Co and 192Ir sourcesin HDR brachytherapyStefan Strohmaier, MSc1, Grzegorz Zwierzchowski, MSc, PhD2

1University of Applied Sciences, Technikum Wien Medical Engineering, Vienna, Austria, 2Department of Medical Physics, Greater Poland

Cancer Centre, Poznan, Poland

Abstract

This paper compares the isotopes 60Co and 192Ir as radiation sources for high-dose-rate (HDR) afterloadingbrachytherapy. The smaller size of 192Ir sources made it the preferred radionuclide for temporary brachytherapy treat-ments. Recently also 60Co sources have been made available with identical geometrical dimensions. This paper com-pares the characteristics of both nuclides in different fields of brachytherapy based on scientific literature. In an addi-tional part of this paper reports from medical physicists of several radiation therapy institutes are discussed. The purposeof this work is to investigate the advantages or disadvantages of both radionuclides for HDR brachytherapy due to theirphysical differences. The motivation is to provide useful information to support decision-making procedures in the

selection of equipment for brachytherapy treatment rooms. The results of this work show that no advantages or dis-advantages exist for 60Co sources compared to 192Ir sources with regard to clinical aspects. Nevertheless, there are poten-tial logistical advantages of 60Co sources due to its longer half-life (5.3 years vs. 74 days), making it an interesting alter-native especially in developing countries.

J Contemp Brachyther 2011; 3, 4: 199-208

DOI: 10.5114/jcb.2011.26471

Key words: 60Co, 192Ir, HDR brachytherapy, radionuclides.

Review articles

Received: 14.06.11

Accepted: 12.12.11Published: 31.12.11

Purpose

The use of high-dose-rate (HDR) afterloading brachy-

therapy is a highly widespread practice today. It hasproven to be a successful treatment for cancers of theprostate, cervix, endometrium, breast, skin, bronchus,oesophagus, head and neck and several other types of can-cer. Up to now the production of small sources for HDRafterloading was only possible for iridium-192, because oftechnological reasons. The smaller size of the sourcesallowed interstitial treatment and optimization of dose.This made 192Ir the most used isotope for HDR afterload-ing brachytherapy worldwide. Recently 60Co sourcesbecame available with identical geometrical dimensions asminiaturized 192Ir sources. These two radionuclides showdifferent physical characteristics, while 60Co offers logisti-cal and economic advantages. Typical intervals for source

replacements foresee 25 source exchanges for 192Ir com-pared to just a single one for 60Co, resulting in reducedoperating costs as a result of a longer half-life, 5.3 years(60Co) compared to 74 days (192Ir). Especially in develop-ing countries frequent source exchanges may be challeng-ing in view of the available infrastructure.

Radionuclides in brachytherapy

Many radionuclides have a history as a source forbrachytherapy, but today only a few are commonly used.

They are characterized by the rate at which their strengthdecays (half-life), by how much radioactivity can be obtainedfor a given mass of the radioactive source (specific activity)

and by the energies and types of the radiation particles thatare emitted from the source (energy spectrum) [1].

Half-life

The strength of a radiation source decays exponentially.The half-life is the time it takes for the source strength todecay to half of its initial value. The half-life of a specificsource determines how long it can be used over a period oftime. Sources with short half-lives can reduce the risks ofradiation exposure around the patient, because the radia-tion decreases rapidly in time. When it comes to dose cal-culation the decay of the source may not be explicitlyaccounted for if the source has a long half-life. For example,137Cs sources, with a half-life of 30 years, are assumed tokeep constant source strength during the treatment periodof a few days, whereas the dose calculation for 125I sources,with a half-life of just 59.49 days, needs to consider the decayof the sources during the time of the implant. The activityof an isotope is defined as its decay per unit of time.

Specific activity

The specific activity is the ratio of activity containedwithin a unit mass of the source. The strength of a brachy-


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Stefan Strohmaier, Grzegorz Zwierzchowski200

therapy source for practical applications is limited by itsspecific activity. When a parent nuclide is activated ina neutron flux field, the number of radioactive nuclides perunit mass that may be obtained is limited by the neutronflux field strength, the parents nuclide neutron cross-sec-

tion, and the sources half-life. This is important for HDRinterstitial brachytherapy applications, which require smallsource dimensions as well as high source strengths. 192Irsources are very popular because of its high specific activ-ity and high neutron cross-section [2].

Energy

The average energy of a brachytherapy source deter-mines the penetrability of the photon particles emittedfrom the source. High energy sources allow a higher doseto tissues at larger distances to the sources, but requirethicker shielding for protection of the staff. Low-energyphoton emitting sources, such as 125I and 103Pd, are often

used for permanent brachytherapy treatments, becausethe radiation of these sources is mostly diminished by thepatients tissue. When high-energy sources, such as 198Au,are used, the patient needs to stay in the hospital until thesource strength decays to a suitable value, such that theradiation exposure outside the patient satisfies state regu-lations [3]. The thickness of any materials that reduce theexposure by 50% is known as the half-value layer (HVL)[4]. In general, intracavitary irradiation requires more pene-tration in tissue than interstitial brachytherapy. Sourcesadequate for intracavitary applications should have radialdose functions which do not fall rapidly with distance.137Cs, 192Ir and 60Co would be suitable candidates, becauseall of them emit relatively high-energy photons [5].

Dosimetry in brachytherapy

Accurate determinations of dosimetric characteristics ofbrachytherapy sources are very important. The dosimetriccharacteristics can be determined by using appropriateexperimental techniques and dosimeters. Theoretical cal-culations are also very useful in the determination of dosi-metric characteristics of brachytherapy sources [6, 7].

Experimental techniques

Dose measurement in brachytherapy requires accuratelypositioned detectors, which produce measurable signals

that have known relationship to the relative or absolutedose in the medium in the absence of the detector. Detec-tors commonly used in brachytherapy are thermolumi-nescent detectors (TLDs), radiochromic films, plastic scin-tillators and thermoluminescent (TL) sheets. The dosedistribution of brachytherapy sources is characterized bylarge dose gradients and a large range of dose rates. A suit-able detector must have a wide dynamic range, flat ener-gy response, small size and high sensitivity. Another chal-lenge is the variation of detector response with a changein source to detector distance [6-10].

AAPM TG43 algorithm

The dose distribution in the vicinity of a brachytherapysource is a complex function of the emitting energy spec-

tra, the geometry of the source and the material close tothe source. The most established dosimetry protocol fordose calculation in brachytherapy has been elaborated bythe American Association of Physicists in Medicine(AAPM) for the dosimetry in the vicinity of radiation

sources. Most reviews on newly introduced radionuclidesare based on this dosimetry method. The TG-43 approachconsists of using measured and Monte Carlo generateddose-rate distributions directly for clinical dose calculationaided by a standard table-lookup formalism. Today mosttreatment planning software vendors have implementedthe TG-43 formalism and the recommended parameters intheir systems [11-13]. The radial dose fall-off from any gam-ma source, the rate at which dose decreases with increas-ing distance from the source, is determined by:

Geometry factors

For a point source of radiation, the dose decreases ap-

proximately as the square of the distance from the source.Radioactive sources used in brachytherapy are not pointsources; they are typically linear sources. For these linearsources the decrease in dose falls more as the first power ofthe distance, rather than the square of the distance.

Attenuation

The dose rate also decreases with increasing distancefrom the source, because of attenuation of the gamma raysby the intervening tissues or environment [14].

General 2D formalism

The dose rate D(r,

) of a cylindrically symmetric source X(X = L for a line source and X = P for a point source) at anarbitrary point F(r,) follows the following equation:

Gx(r,)D(r,) = Sk ------------ gx(r) F(r,)

Gx(ro,o)

r distance (in centimetres) from the centre of the activesource to the point of interest,

r0 denotes the reference distance which is specified to be1 cm,

polar angle specifying the point of interest, P r, , rela-tive to the source longitudinal axis, 0 reference plane

defines the source transverse plane, and is specifiedto 90,

Sk source strength specified in air kerma strength, dose rate constant defined as the ratio of the dose rate

at the reference point; the dose rate constant can beevaluated by using a calculation method such as theMonte Carlo method,

Gx(r,) geometry factor that describes the effect of theactive source material distribution within thesource on the dose distribution outside the source,

gx(r) radial dose function accounting for the effects ofabsorption and scatter on the dose distribution inthe medium along the transverse axis of the source,

F(r,) anisotropy factor describes the effects of anisotrop-ic photon attenuation, by the source material or


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Comparison of 60Co and 192Ir sources in HDR brachytherapy 201

materials away from the source transverse axis.Geometric factors may cause the dose to be direc-tion-dependent [15] (Fig. 1).

Experimental dosimetry versus Monte Carlo

The main disadvantage of experimental dosimetry is thepositioning of the detectors near the sources (distances lessthan 1 cm from the source). The high dose gradient thatappears near the source and the poor signal-noise ratio forgreat distances away from the source affect the accuracy. Onthe other hand, Monte Carlo calculation accuracy can beaffected by inaccuracies in the geometric configuration of thesource, uncertainties in the cross-sections and the modellingof the physical processes in the Monte Carlo code. MonteCarlo calculations can be verified by measuring a set of pointswhere experimental uncertainties are relatively low. Animportant aspect is how to validate the comparison of MonteCarlo with measurements. For non-low energy radionuclides

such as 60Co and 192Ir, it is accepted that results can be vali-dated by comparison with experimental results for a sourceof the same radionuclide and similar design. But for lowenergy sources it is recommended to do the validation forthe same source model, because the dependence on sourcegeometry and materials is very significant. Further verifica-tion of the geometric data of the source provided by the ma-nufacturer is recommended. This can be done by contactradiography, special cameras or microscopy [6-8, 15].

60Co and192Ir sources in HDR afterloadinggynaecological brachytherapy

Richter et al. compared a 60Co and a 192Ir source of iden-

tical dimension and construction and discussed tissueabsorption, geometry function and integral dose of the twosources. The cobalt source consists of a metallic 60Co cylin-der with a length of 3.5 mm and a diameter of 0.6 mm.The source is surrounded by a cylindrical steel jacket andan outer diameter of 1 mm [16]. The results of the calcula-tion of the differences between the two sources are shownin Fig. 2. For instance, the dose for 60Co sources in fat tis-sue is 0.4 percent higher and 0.8 percent lower for the rec-tum than for 192Ir sources. The largest difference is foundin lung tissue, with 2.1%. The biological response diffe-rence to the different energies of the 60Co and 192Ir sourcesare therefore negligible [16].

Figure 3 shows the anisotropy factor () as in practiceparameterization of the rdependence is introduced for sim-plification and the anisotropy factor F(r,) is reduced to() at the distance r= 4 cm. The 60Co source shows advan-tages as the 192Ir source shows deviations at the top of thesource ( = 0) and at the source mounting ( = 180) [16].

The radial dose function depends on the phantom size.Figure 4 shows the results for both line sources in an infi-nite phantom. Like in an ideal point source, the initial fallof the radial dose function of the iridium source is lesssteep than the cobalt source. This relation reverses at dis-tances greater than 22 cm as a result of the higher photonenergy of 60Co. This also explains that although the inte-

gral dose in the patient is slightly lower, higher roomshielding is required for the use of 60Co sources in bra-

chytherapy. The influence of the phantom size on the ra-dial dose is shown in Fig. 5; near the surface of the spheri-

cal phantom with the radius of 15 cm a reduction of theradial dose is clear [16].

Fig. 1. Encapsulated line source, illustrating calculation of

the dose rate at point P(r,) = P(x,y) relative to the source

centre. The source length and encapsulation thickness are

denoted by L and t [15]

z

P(r,)

21

r

t L

xP(r0,

0)

r0 = 1 cm

Fig. 2. Differences in absorbed dose of several tissues fora 60Co source in comparison to a 192Ir source [16]

Differences(%)

2.5

2.0

1.5

1.0

0.5

0

0.5

1.0

1.5

Adipose tissue

Body fat

Connective tissue

Liver

Lung 0.26

Lung 1.05

Prostate

Rectum

Tongue

Fig. 3. Anisotropy factor () for the 60Co and 192Ir source

[16]

0 20 40 60 80 100 120 140 160 180

Polar angle ()

Co-HDR

Ir-HDR

Anisotropy factor

Arb.units

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.20.1

0


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Richter et al. also show an almost identical dose distri-bution for the reviewed 60Co and 192Ir sources. In Fig. 6 anequal dose distribution for a combination of a ring-shapedand linear applicator is demonstrated [16].

Park et al. [17] also compared the dose distributions of

three radiation sources and two treatment planning sys-tems in HDR intracavitary brachytherapy. In Figs. 7 and 8the radial dose function and the anisotropy function usedfor the study of the HDR sources are shown. Figure 7shows that the influence of different phantom size and thephysical difference between 60Co and 192Ir are distingu-ished from those of the geometrical difference on the radialdose function. Figure 8 shows that the effect of a physicaldifference is also bigger than that of a geometrical diffe-rence. High penetration of gamma rays emitted from 60Corepresents a wide difference in the source tip and theregion connected to the source cable in this figure.

Dose at large distances from 60Co and192Irbrachytherapy sources

Ionization chamber measurements were performed ina water tank to describe the decrease of dose with distance

of the mentioned sources. As an independent check of theresults, Monte Carlo calculations with the EGS-4 codesystem were executed in a simulated water phantom.The dimensions of the water tank were changed to simu-late different patient sizes. The decrease of the dose around

a brachytherapy source is mainly described by the inversesquare law. Within the first few centimetres this is accu-rate. But at 10 cm deviations from the inverse square lawmay be as large as 20%, depending on the energy of theemitted radiation. The relationship between the tissueattenuation factor T(r) and the radial dose functiong(r) isdiscussed in a report of the AAPM Radiation TherapyCommittee Task Group No. 43 [18]. For point source geo-metry and energies > 300 keV, there is a very close rela-tionship between the two functions, so thatg(r) equals T(r)normalized at the distance of r0 = 1 cm.

Venselaar et al. [19] investigated three isotopes: 60Co,192Ir and 137Cs. Only the results for 60Co and 192Ir are con-

sidered, because137

Cs is not of relevance in HDR bra-chytherapy anymore. Both 60Co and 192Ir have a long his-tory in intracavitary brachytherapy. The sources used inthe study consisted of two 60Co sources with a total activ-ity of 89 GBq and a 192Ir source with a nominal activity of

Arb.units

1.0

0.9

0.8

0.7

0.6

0.50.4

0.3

0.2

0.1

0

0 2 4 6 8 10 12 14 16 18 20

Distance r(cm)

Co-HDR

50 cm phantome

15 cm phantome

Ir-HDR

Radial dose function

Fig. 5. Radial dose functions for greater distances from the

source in an infinite phantom [16]

Arb

.units

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

0 10 20 30 40 50 60 70 80 90 100

Distance r(cm)

Co-HDR

Ir-HDR

Radial dose function

Fig. 4. Radial dose function of 60Co and 192Ir source [16]

Fig. 6. Dose distribution for60

Co and192

Ir source of a ring-shaped and linear applicator for the irradiation of a cervical carci-noma (upper half depicting 192Ir, lower half 60Co) [16]


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370 GBq. The measurement set-up consisted of a watertank with dimensions of 100 50 35 cm3 in whicha source holder and an ionization chamber could be movedindependently. The distance to the ionization chambercould be varied up to 60 cm. The influence of smaller phan-tom size was studied by lowering the water level and plac-ing foam blocks inside the tank. This allowed the width ofthe phantom to be altered from 18 to 50 cm. Absolute dosewas measured with two ionization chambers while measu-rement time varied from 60 s for short distances to 600 s atlarger distances. Measurements were repeated to deter-mine the reproducibility of the set-up. The detector wasplaced at the perpendicular bisector of the sources to avoidanisotropy effects. In all cases the sources were consideredto reflect point source configuration. All measurementswere done both in air and water to calculate T(r). For the

Monte Carlo simulation the user code DOSRZ was used,which assumes a cylindrical geometry. It was used to cal-culate the energy deposited in cylindrical detectors, com-posed of water and situated at the axis of a large cylindri-cal phantom, the point sources being located at the sameaxis. A 20 cm radius was defined for the phantom, a 10 cmthick slab behind the source ensured backscattered radia-tion, while a 10 cm thick water slab at the opposite side ofthe simulated phantom ensured full scatter conditions [19].

Dose as a function of distance, D(r) was converted intoT(r) using the inverse square law and normalization to uni-ty at the distance of 1 cm. In Fig. 9 the Monte Carlo resultsare shown for both the 192Ir and 60Co isotope, in the rangeof 10-60 cm. In Fig. 10 the results of the measurements andthe calculations for an infinite sized phantom are present-ed together for all three investigated isotopes. The influ-

Fig. 7. Radial dose functions of the different HDR sources.

The radial dose function of the MicroSelectron classic 192Ir

source was modified to simulate a similar phantom condi-

tion as the 60Co source [17]

0 2 4 6 8 10 12 14 16 18 20

Distance (cm)

microSelectron classic 192Ir BEBIG new 60Co

Modified microSelectron 192Ir

1.05

1.00

0.95

0.90

0.85

0.80

0.75

0.70

0.65

0.60

Radialdosefu

nctiong(r)

Varian new 192Ir

Fig. 8. Anisotropy functions for the different HDR sources

at the distance of (A) 0.5 cm, (B) 1 cm and (C) 5 cm [17]

1.00

0.75

0.50

0.25

0

0.25

0.50

0.75

1.00

The polar angle is defined relative to source cable

microSelectron classic 192Ir

Varian new 192Ir

BEBIG new 60Co

0

A

30

60

90

120

150180

030

60

90

120

150180

030

60

90

120

150180

B C

0 5 10 15 20 25 30 35 40 45 50 55 60

r(cm)

193Ir

Tissue attenuation factor

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

0 5 10 15 20 25 30 35 40 45 50 55 60

r(cm)

MC model Meisberger

60Co


1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

Fig. 9. Results of the Monte Carlo calculations for the 192Ir and 60Co isotope in the range of 10 < r 60 cm. In the range of 0 r

10 cm, results are presented from the model of Meisberger. The lines represent a fit through the Meisberger and the MonteCarlo data points, according to the model of Kornelsen [19]

T(r)

T(r)

model KomelsenMC model Meisbergermodel Komelsen


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ence of the dimensions of the water phantom was deter-mined by lowering the water level and reducing the widthwith foam blocks. An extreme reduction of the widthfrom 50 to 18 cm reduced the local dose by 5.4% at 15 cmsource-to-detector distance and by 13.3% at 55 cm source-to-detector distance, in the case of the 60Co measurements.

In the case of the 192Ir measurements, a maximum reduc-tion in the local dose of 14% was found, when the sourcewas brought from the centre of the phantom to 5 cm belowthe water surface. In all cases in which the dimensions ofthe phantom or the position of the source and detectorwere varied, reductions of this magnitude were found,compared with the measurements in the infinite phantom.From this it can be concluded that the results from themeasurements in the large water phantom may be used todescribe T(r) for a wide range of situations. The accuracyof the measurements is determined by the accuracy withwhich the distance in the water phantom could be meas-ured. Venselaar et al. conclude the same for both isotopes

under consideration in the range of 10-60 cm from thesource. A typical value of T(r) for a distance of r= 25 cmfor all sources is approximately 0.5 and for r= 50 cm, T(r)is in the order of 0.15. T(r) curves prove to be smooth func-tions for both isotopes.

Dose rate distributions around modern 60Co and192IrHDR sources

As recommended in the updated TG-43 report byAAPM, dose distribution data of brachytherapy sourcesshould be obtained experimentally or by the Monte Carloalgorithm. These data can then be used as input in HDRtreatment planning systems. Granero et al. used the Monte

Carlo method to obtain the dose rate distribution of a mo-dern 60Co and 192Ir afterloading source [20].

Dose rate distribution around60Co source

Granero et al. investigated the dose rate distribution ofa 60Co HDR source by the Bebig company (modelCo0.A86) using the Monte Carlo code GEANT4 [20]. The

new Bebig60

Co is very similar to the old Bebig source(model GK60M21), but the new source has a smaller activecore (0.5 mm in diameter vs. 0.6 mm) and a more round-ed capsule tip. It is composed of a central cylindrical activecore made of metallic 60Co, 3.5 mm in length and witha diameter of 0.5 mm. The active core is surrounded bya cylindrical 0.15 mm thick stainless-steel capsule with anexternal diameter of 1 mm.

The source was located in the centre of a spherical waterphantom of 50 cm in radius, which acts as a phantom forthe 60Co source up to a distance of 20 cm from the source.The density used for the liquid water was 0.998 g/cm3 at22C as recommended in the TG-43 report of the AAPM.To obtain the along and away dose rate table, a grid sys-tem composed of 400 800 cylindrical rings 0.05 cm thickand 0.05 cm high, concentric to the longitudinal source axis,was used. A system of 400 180 concentric spherical sec-tions 0.05 cm thick with an angular width of 1 in the polarangle was used to obtain the dose rate distribution in theform given by the TG43 report. The 60Co source was locat-ed in the centre of a 4 4 4 m3 air cube to estimate air-kerma strength. Kerma was scored using cylindrical ringcells, 1 cm thick and 1 cm high, located along the trans-verse source axis.

To obtain the TG-43 dosimetric parameters from D(r,)the geometric factor GL(r,) with a length of L = 3.5 mm hasbeen used. The dose rate constant obtained was = 1.087

0.011 cGyh1U1, where the air kerma strength isexpressed in units of 1 U = 1 Gyh1m2. The radial dosefunction, GL(r,) is shown in Table 1 [20]. The dose rate dis-tribution obtained for the new Bebig source has been com-pared with that obtained for the old Bebig HDR source;the results are depicted in Fig. 11. This comparison showsthat both dose rate distributions are almost identical infront of the source; the difference between the sources isless than 0.5%.

Dose rate distribution around192Ir source

In a different study Granero et al. investigated the doserate distribution in liquid water media for a 192Ir HDR

source (type Ir2.A85-2). The source consists of a cylindri-cal pure iridium core, 3.5 mm in active length with a dia-meter of 0.6 mm. The source is surrounded by a capsulemade of stainless steel 316 L and is very similar to otherexisting HDR sources. Again the Monte Carlo methodwas used to obtain dose rate distributions, using theGEANT4 code.

The source is immersed in the centre of a 40 cm radiuswater phantom, which acts as an unbounded phantom upto 20 cm of radial distance. Two different grid systems havebeen applied to score the kerma. The first one is used toobtain the dose rate in the form of along and away tables,D(y,z); it is composed of 400 80 cylindrical rings with

a height and width of 0.05 cm. The second grid system isused to obtain the dose distributions in polar coordinates

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

00 5 10 15 20 25 30 35 40 45 50 55 60

r (cm)

Ir-192 (MC) Cs-137 (MC) Co-60 (MC)

Ir-192 (meas) Cs-137 (meas) Co-60 (meas)


T(r)

Fig. 10. Results of the measurements and the calculations

for an infinite sized phantom, for 192Ir, 137Cs and 60Co.

Results in the range of 10-60 cm obtained by measurement

are presented as points. Lines are used for the Monte Car-

lo results [19]


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D(r,). The air-kerma strength has been calculated to nor-malize the dose rate distributions in water. Therefore theHDR source was placed in a 4 4 4 m3 air volume, beingin composition as recommended in the updated TG-43report of the AAPM. Air-kerma was scored along the trans-verse axis of the source using cylindrical cells, 1 cm thickand 1 cm high.

To obtain the TG-43 dosimetric parameters from D(r,)the geometric factor GL(r,) with a length of L = 3.5 mmwas used. The dose rate constant obtained was = 1.109 0.011 cGyh1U1, where the air kerma strength isexpressed in units of 1 U= 1 Gyh1m2. The radial dosefunction GL (r,) is presented in Table 2. The dose rate dis-

tribution obtained for the Bebig HDR source has been com-pared with that obtained for the Bebig PDR source;the results are depicted in Fig. 12.

Shielding

Gamma radiation from a 60Co source is more pene-trating than radiation from 192Ir and therefore differentthicknesses of the same material are required to obtainidentical shielding results. The higher the energy ofa source, determining the penetrability, the thickerthe shielding material for protection of the staff must be.The following equation is commonly used to calculatethe thickness required from shielding materials to atten-

uate radiation to one half (the half value, HVL) or to onetenth (the tenth value layer, TVL). The half-value layers

of various materials for 60Co and 192Ir are listed in Table 3[22-24].

Instantaneous dose rates around HDR brachytherapyunits with 370 GBq sources (192Ir) made their use in con-ventional rooms not possible. HDR afterloading machines

require special HDR facilities, which have to provide satis-factory radiation protection [25, 26].


Distance r (cm) GL(r)0.25 1.007

0.5 1.036

0.75 1.015

1 1

1.5 0.992

2 0.984

3 0.968

4 0.952

5 0.936

6 0.919

7 0.902

8 0.884

10 0.849

12 0.813

15 0.756

20 0.665

Table 1. Radial dose function for the Bebig 60Cosource (model Co0.A86) between 0.25 and 20 cm

[20]

Distance r (cm) GL(r)0.25 0.99

0.5 0.996

0.75 0.998

1 1

1.5 1.002

2 1.004

3 1.005

4 1.003

5 0.999

6 0.991

7 0.981

8 0.968

10 0.935

12 0.984

15 0.821

20 0.686

Table 2. Radial dose function for the Bebig HDR192Ir source (model Ir2.A85-2) between 0.25 and

20 cm [21]

Fig. 11. Isodose curves in cGyh-1 U-1 for the old and the

new Bebig HDR source. The background is the ratio

between the kerma rate distribution [20]

5

4

3

2

1

0

1

2

3

4

5

1.12

1.11

1.10

1.09

1.08

1.07

1.06

1.05

1.04

1.03

1.02

1.01

1.00

0.99

0.980 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

y (cm)

K(y,z) (cGyh1U1) and KNew/KOld(y,z)


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Experiential reportsDuring the literature research for this work, talks to

individual physicists of several radiation therapy instituteswere conducted. Those included institutes which havealready. Interviewed institutes included all radiation the-rapy facilities using 60Co in Germany currently and sev-eral 192Ir users in Austria. All German brachytherapy unitscurrently equipped with 60Co used 192Ir in the past. Ofthose, every discussion partner mentioned that financialmotives were the initial reason for a change of the radionu-clide as fewer source exchanges are required using 60Co.Though no clinical results for miniaturized 60Co sourcesare available yet, everyone pointed out that the essential

dose distributions of these isotopes are similar referring toavailable studies and the same clinical results are expect-ed [9, 16, 24]. Also easier handling and quality assurance

due to fewer source exchanges is widely appreciated. Inmost cases it was not necessary to extend radiation pro-tection, because the therapy rooms already met radiationprotection directives. One institute needed to make a deve-lopment by adding an additional lead, but still calculated

a return on investment after 10 years. In addition, not a sin-gle conversation partner said they would change back to192Ir if funding would allow it now, but also mentionedthat if there were no financial influence on the decisionmaking process it would not have been considered tochange the familiar system.

The situation in Austria is different, as not in a singlefacility is a 60Co source in operation. Many institutes areused to working with well-established 192Ir afterloadingbrachytherapy units. Though aware of the logistical advan-tages, the majority do not consider changing the existingsystems. In two cases it had been contemplated to swap to60Co sources but it was not carried out, because rebuild-

ing would have been required to provide adequate radia-tion protection. One institute is planning to build newradiation therapy rooms and pointed out that the possi-bility to use 60Co in the future due to financial and logisti-cal advantages will be highly considered. During all theseconversations it also became obvious that the isotope is notthe main basis of decision making. The equipment of theafterloader also influences the reason to purchase froma vendor, as physicians may prefer specific applicatorswhich are not in the portfolio of the afterloader from a dif-ferent company or might have gained years of experienceon products of certain manufacturers. The administrativeeffort to deal with an increased exchange in sources is alsooften very well automated due to great experience and is

not regarded as a burden. Thanks to many thoughts on thistopic from physicists of several institutes, it cannot beassumed that the motivation to change existing systemsjust out of potential financial benefits is always present;much more decisive might be the satisfaction with accus-tomed systems.

D(cGyh1U1)

Fig. 12. Isodose curves in cGyh-1 U-1 for the HDR and the

PDR source [21]

5

4

3

2

1

0

1

2

3

4

50 1 2 3 4 5

y (cm)

z(cm)

HDR PDR

Shielding thickness192Ir 60Co

Shielding material HVL TVL HVL TVL

Lead 4.83 16.26 12.45 41.15

Steel 15.5 50.8 22.081 73.66

Concrete 48.26 157.48 66.04 218.44

Aluminium 48.26 157.48 66.04 218.44

Table 3. Half-value layers and tenth-value layersfor 192Ir and 60Co isotopes [24]

Brachytherapy department SourceClinical centre Bayreuth, Germany 60Co

Clinical centre Deggendorf, Germany 60Co

Clinical centre Neumarkt, Germany 60Co

Clinical centre Nrnberg Nord, Germany 60Co

County hospital Gummersbach, Germany 192Ir

University hospital Wrzburg, Germany 60Co

Clinical centre Krems, Austria 192Ir

County hospital Klagenfurt, Austria 192Ir

Hospital Hietzing Rosenhgel Vienna, Austria 192Ir

University hospital Graz, Austria 192Ir

Hospital Kaiser-Franz-Josef Vienna, Austria 192Ir

Hospital SMZ-Ost Vienna, Austria 192Ir

University hospital general hospital Vienna, Austria 192Ir

Table 4. Source used in interviewed brachytherapydepartments


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Conclusions

Both 60Co and 192Ir isotopes have a long history in thefield of brachytherapy. Several hundred HDR afterloadingunits equipped with 60Co or 192Ir sources have been put into

use in the past. At the beginning of HDR brachytherapy60Co was used in the form of pellets. 192Ir achieved wideacceptance as the first remote afterloading systems wereintroduced. The possibility of manufacturing miniaturizedsources allowed HDR brachytherapy to be applied to inter-stitial techniques and led to a high market preference for192Ir. The majority of temporary interstitial and intracavi-tary treatments in the western world now appear to be deliv-ered by 192Ir. Still, 60Co has been widely deployed in Asiaand especially in Japan for the past decades. Since 60Co isalso available in miniaturized form with geometrical dimen-sions identical to those of 192Ir sources, it seems that there ispotential to change market share. However, the higher emit-ting energy of 60Co photons requires increased radiation

protection. The main advantage of considering 60Co forHDR brachytherapy applications is connected with the longhalf-life (t1/2 = 5.27 years vs. 74 days) with source exchangesin only intervals of a few years. Despite the fact that 60Coand 192Ir sources of identical shape and dimensions havedifferent physical characteristics, they show identical dosedistributions, as demonstrated in various studies. Severaldosimetric and theoretical studies support clinical equiva-lence of modern HDR. Differences in dose absorption in dif-ferent tissues are insignificant as well. It can be assumed thatthere are no advantages or disadvantages of 192Ir comparedto 60Co. However, simulations show that due to the higherphoton energy of 60Co beyond a distance of 20 cm from

the source, the integral dose is higher compared to 192Ir.The higher energy requires more room shielding for theapplication of a 60Co source. Many existing therapy roomswould need reconstruction regarding radiation protection.Besides, only a single company offers miniaturized 60CoHDR sources currently requiring their afterloader. The issuesof reconstruction and the change of existing afterloadingunits will therefore not establish 60Co in the future as noclinical advantages exist. Nevertheless, there is big poten-tial for 60Co as an alternative to 192Ir for newly constructeddepartments where the selection of the equipment is still inprogress. The prospect of reduced costs due to fewer sourceexchanges will be of great interest if financial circumstanceshave to be considered. There may be great potential in deve-loping countries where lots of radiation therapy facilitiesare still being planned. Especially the lack of experiencedpeople for source exchange, disposal and quality assuranceplays an important role. 25 source exchanges are requiredfor 192Ir for one exchange of a 60Co source. In countrieswhere foreign personnel must conduct these responsibili-ties it is even more costly. These obvious logistical advan-tages and potential savings will encourage other companiesto make miniaturized 60Co sources available in the nearfuture.

Acknowledgments

Special thanks to Dipl. Ing. Peter Tonhuser for his help,factual material and for substantive care.

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comparison of sources hdr -- jcb-3-17925

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